Precipitated silicas, a process for their preparation and their use in vulcanizable rubber mixtures

ABSTRACT

Precipitated silicas, characterized in that they have a CTAB surface area (in accordance with ASTM D 3765-92) of 200 to 400 m 2  /g, a DBP index (in accordance with ASTM D 2414) between 230 and 380 ml/100 g as powder and 180-250 g/100 g as granulate, a silanol group density (V 2  -NaOH consumption) of 20 to 30 ml and the following macropore size distribution which is typical of the surface area range involved, determined by means of Hg porosimetry (DIN 66 133) for specific pore size intervals (incremental mode of application): 
     
         ______________________________________
 
    
               CTAB surface
                   CTAB surface
                              CTAB surface
          area range:
                   area range:
                              area range:
          200-250 m 2  /g
                   250-300 m 2  /g
                              300-400 m 2  /g
Pore size interval  nm!
            Hg consumption in ml/g of silica
______________________________________
10-20       0.27-0.49  0.35-0.50  0.32-0.42
20-30       0.22-0.32  0.15-0.30  0.17-0.22
30-40       0.15-0.21  0.12-0.17  0.12-0.15
40-50       0.11-0.16  0.09-0.12  0.08-0.11
50-60       0.08-0.12  0.06-0.10  0.06-0.09
______________________________________

This is a divisional of application Ser. No. 08/655,035 filed on May 29, 1996 now U.S. Pat. No. 5,705,137.

INTRODUCTION AND BACKGROUND

The present invention relates to precipitated silicas used as reinforcing fillers in vulcanizable rubber mixtures. Another aspect of the invention resides in the process for preparing certain precipitated silicas. A still further aspect of the invention resides in rubber formulations containing the precipitated silicas.

Precipitated silicas, in general, are known as is their use in tire formulations as shown by the following references.

S. Wolff, E. H. Tan: Performance of Silicas with Different Surface Areas in NR. Paper presented on the occasion of the ACS Meeting, New York City, N.Y., April 1986; and,

S. Wolff, R. Panenka, E. H. Tan: Reduction of Heat Generation Truck Tyre Tread and Subtread Compounds. Paper presented on the occasion of the International Conference on Rubber, Jamshedpur (India), Nov. 1986.

An essential parameter for characterizing a precipitated silica is the surface area, which is determined either by the adsorption of nitrogen (ISO 5794/1, Annexe D) or, of greater relevance to rubber engineering, by the adsorption of CTAB in accordance with ASTM D 3765-92. With reference to the rubber engineering data obtained, precipitated silicas are classified in six surface area classes in accordance with ISO 5794/1.

When applied to tires, however, almost exclusively precipitated silicas with CTAB surface areas between 100 and 200 m² /g are used. See U.S. Pat. No. 5,227,425 and S. Wolff: The Influence of Fillers on Rolling Resistance, presented at the 129th meeting of the Rubber Division of the American Chemical Society, New York, N.Y., Apr. 8-11, 1986.

The reason for this is that precipitated silicas for use in tires must have high strength values and, especially in the region of the tread surface, must have good abrasion resistance. This can only be ensured by using precipitated silicas with surface areas in the range mentioned above. At present, precipitated silicas with CTAB surface areas >200 m² /g, on the other hand, are hardly ever used. However, it is precisely these precipitated silicas which should be characterized by a particularly high abrasion resistance and thus are of particular interest for practical application in tires. The essential reason why they are not used is due to the fact that precipitated silicas with increasing surface areas are extremely difficult to distribute (disperse) in a rubber mixture. This low capacity for being dispersed means that these products do not provide the general properties expected of them and thus offer no advantages, and sometimes even offer disadvantages, as compared with the precipitated silicas currently used in tires.

The reason for this poor dispersion of high surface area precipitated silicas is based on another important property of precipitated silicas, and that property is their structure (measured by means of DBP absorption in accordance with ASTM D 2414). Particularly important, however, is the distribution of macropore sizes (measured using Hg porosimetry in accordance with DIN 66 133) and how it changes with surface area; see the paper by U. Gorl, R. Rausch, H. Esch and R. Kuhlmann entitled: Kieselsaurestruktur und ihr Einfluβ auf das gummitechnische Wertebild, presented on the occasion of the German Rubber Conference in Stuttgart, June 1994.

In other words, the spread of macropore sizes generally decreases with increasing surface area, if it is not possible to produce additional large pores by means of appropriate precipitation techniques during the production of precipitated silica with the same surface area.

In particular, precipitated silicas with CTAB surface areas >220 m² /g require special precipitation techniques in order for them to have sufficiently large macropores, and thus good dispersion behavior, despite the large surface area.

It is known that precipitated silicas with the following physico-chemical parameters can be used in rubber mixtures (DE-A 44 27 137):

    ______________________________________     BET surface area      35 to 350 m.sup.2 /g     Ratio of BET/CTAB surface areas                           0.8 to 1.1     Pore volume PV        1.6 to 3.4 ml/g     Silanol group density 6 to 20 ml     (V.sub.2 = NaOH consumption)     Average size of aggregate                           250 to 1500 nm     CTAB surface area     30 to 350 m.sup.2 /g     DBP index             150 to 300 ml/100 g     V.sub.2 /V.sub.1 by Hg porosimetry                           0.19 to 0.46     preferably            0.20 to 0.23     DBP/CTAB              1.2 to 2.4     ______________________________________

An object of the invention therefore is to obtain precipitated silicas with CTAB surface areas of >200 m² /g which are characterized by a particularly high distribution of macropore sizes and thus by particularly good dispersion in a rubber mixture.

Another object of the invention is to obtain improved rubber formulations and products.

SUMMARY OF THE INVENTION

In attaining the above and other objects, one feature of the invention is a precipitated silica product which has a CTAB surface area (in accordance with ASTM D 3765-92) of 200 to 400 m² /g, a DBP index (in accordance with ASTM D 2414) between 230 and 380 ml/100 g, a silanol group density (V₂ -NaOH consumption) of 20 to 30 ml and the following macropore size distribution which is typical of the surface area range involved, determined by means of Hg porosimetry (DIN 66 133) for specific pore size intervals (incremental mode of application):

    ______________________________________               CTAB surface                        CTAB surface                                   CTAB surface               area range:                        area range:                                   area range:               200-250 m.sup.2 /g                        250-300 m.sup.2 /g                                   300-400 m.sup.2 /g     Pore size interval  nm!                 Hg consumption in ml/g of silica     ______________________________________     10-20       0.27-0.49  0.35-0.50  0.32-0.42     20-30       0.22-0.32  0.15-0.30  0.17-0.22     30-40       0.15-0.21  0.12-0.17  0.12-0.15     40-50       0.11-0.16  0.09-0.12  0.08-0.11     50-60       0.08-0.12  0.06-0.10  0.06-0.09     ______________________________________

The invention also provides a process for producing precipitated silicas having the above defined properties wherein the reaction is carried out in a reaction container of water, heated to 30° to 90° C., preferably 50° to 80° C., the pH of which is adjusted to a pH of 5 to 5.9 by adding small amounts of sulphuric acid, the pH being kept constant at 5 to 5.9 by simultaneous introduction of alkali metal silicate solution and sulphuric acid, with constant shear agitation during the entire precipitating time, precipitation being interrupted for an interval of 30 to 120 minutes to obtain a solids concentration of 40 to 60 g/l. The silica suspension thereby obtained is filtered and washed. The filter cake is subjected to short-term drying, then optionally milled or granulated to produce a finely divided precipitated silica product having the properties set forth above.

Another feature of the invention provides vulcanizable rubber mixtures and use of the silicas according to the invention in vulcanizable rubber mixtures which are characterized in that these rubber mixtures contain 5 to 100 parts, in particular 15 to 60 parts of silicas with respect to 100 parts of rubber.

Silicas according to the invention may be added to the rubber mixture, as a powder or in a low-dust finely divided form; e.g. as granules or microbeads, in the way which is conventional in the rubber industry, in a kneading machine or on rollers. Polymer batches, bales or powders, by means of which the silicas according to the invention are introduced during production of the rubber mixture, may also be used.

A still further feature of the invention resides in precipitated silicas which are characterized in that their surfaces are modified with organosilanes of the formulas I to III

     R.sub.n.sup.1 -(RO).sub.3-n Si-(alk).sub.m -(ar).sub.p !.sub.q  B!(I)

or

    R.sub.n.sup.1 (RO).sub.3-n Si-(alkyl)                      (II)

or

    R.sup.1.sub.n (RO).sub.3-n Si(alkenyl)                     (III)

wherein

B represents --SCN, --SH, --Cl, --NH₂ (if q=1) or --S_(x) -- (if q 2)

R and R¹ represent an alkyl group with 1 to 4 carbon atoms or a phenyl group, wherein all R and R¹ groups may be identical or different,

n is 0, 1 or 2,

alk represents a divalent, straight-chain or branched hydrocarbon group with 1 to 6 carbon atoms,

m is 0 or 1,

ar represents an arylene group with 6 to 12 carbon atoms, preferably 6 carbon atoms,

p is 0 or 1, with the proviso that p and n are not simultaneously 0,

x is a number from 2 to 8,

alkyl represents a monovalent, straight-chain or branched saturated hydrocarbon group with 1 to 20 carbon atoms, preferably 2 to 8 carbon atoms,

alkenyl represents a monovalent, straight-chain or branched unsaturated hydrocarbon group with 2 to 20 carbon atoms, preferably 2 to 8 carbon atoms.

Precipitated silicas modified at the surface with organosilanes may be prepared by treating the precipitated silicas with the organosilanes in mixtures of 0.5 to 50 parts, with respect to 100 parts of precipitated silica.

In a preferred embodiment of the invention, bis-(triethoxysilylpropyl)-tetrasulphane or in a solid form, for example mixed with industrial carbon black, may be used as an organosilane.

Addition of one or more silanes to the rubber mixture may be performed on the one hand simultaneously, together with the silicas according to the invention, wherein reaction between precipitated silica and silane takes place during the mixing process at elevated temperature. On the other hand, addition of silanes to the precipitated silica/rubber mixture may be performed in such a way that the precipitated silica is treated with one or more silanes before it is added to the rubber mixture and the premodified silica introduced to the rubber mixture. For example, this can be performed in the same way as in the procedure shown in DE-A 40 04 781.

The invention also provides rubber mixtures and/or vulcanizates which contain precipitated silicas according to the invention, optionally modified with organosilanes. Precipitated silicas according to the invention may thus be used in all types of rubber which can be cross-linked with accelerator/sulphur, but also with a peroxide, as listed in DIN/ISO 1629. The following may be mentioned by way of example: elastomers, natural and synthetic, oil-extended or not, as individual polymers or as a mixture (blend) with other rubbers such as, for example, natural rubber, butadiene rubber, isoprene rubber, butadiene/styrene rubber, in particular SBR, prepared using the solution polymerization process, butadiene/acrylonitrile rubber, butyl rubber, terpolymers of ethylene, propylene and non-conjugated dienes. Furthermore, the following additional rubbers are suitable for using for rubber mixtures with the rubbers mentioned:

carboxyl rubber, epoxide rubber, trans-polypentenamer, halogenated butyl rubber, rubber made from 2-chlorobutadiene, ethylene/vinyl acetate copolymers, ethylene/propylene copolymers, optionally also chemical derivatives of natural rubber and modified natural rubber.

Also disclosed are conventional further constituents such as rubber carbon black, natural fillers, plasticizers, stabilizers, activators, pigments, antioxidants and processing aids, added in conventional amounts.

Precipitated silicas, with and without silane, may be used in all rubber applications such as, for example, tires, conveyer belts, sealants, V-belts, hoses, shoe soles, etc.

DETAILED EMBODIMENTS OF INVENTION EXAMPLES

The following test methods are used in the examples:

    ______________________________________     Roughness factor F.sup.2 H                          ASTM D 2663-89     ML (1 + 4)           DIN 53 523/524     Tensile strength     DIN 53 504     300% modulus         DIN 53 504     Elongation at break  DIN 53 504     Shore A hardness     DIN 53 505     DIN abrasion         DIN 53 516     MTS data             DIN 53 513     Tear propagation resistance                          DIN 53 507     ______________________________________

The feedstocks used in the application examples are:

    ______________________________________     Trade Name     Description/company     ______________________________________     Buna VSL 1954/S 25                    Styrene/butadiene rubber prepared by                    the solution polymerization process                    (Bayer AG)     Buna CB 11 S   Butadiene rubber (Bayer AG)     Protector G 35 Anti-ozone wax (Fuller)     X 50-S         Bis(triethoxysilylpropyl)-                    tetrasulphane/N 330 50:50 mixture                    (Degussa AG)     ______________________________________

The silane group density is determined using the method given in G. Sears, Analytical Chemistry, 12, 1982-83 (1956).

Determining the distribution of macropores sizes by means of Hg porosimetry

    ______________________________________     Method         DIN 66 133     Apparatus      Autopore II 9220 (Micrometrics GmbH)     Sample preparation                    pre-dry the sample for 15 h/100° C.                    cool to room temperature in a desiccator                    transfer the sample (0.05 + 0.1 g)                    to the penetrometer     Apparatus parameters                    pressure range 4 kPa - 200 MPa                    (53 measuring points)                    angle of contact 140°                    Hg surface tension 0.48 N/m     ______________________________________

Determining the dispersion by means of roughness measurement at the surface of the vulcanizate:

    ______________________________________     Method      ASTM D 2663-89     Apparatus   Surfanalyzer     Procedure   The surface roughness is determined by                 means of a diamond needle moving over                 a freshly cut surface of the                 vulcanizate and the value computed, by                 electronic processing, in the form of                 a roughness factor. The factor is made                 up from the number of signals (F) and                 its intensity (H) as follows:                 Surface roughness factor = F.sup.2 H                 Interpretation of this factor                 indicates that dispersion of fillers                 in the polymer matrix is better the                 lower is this factor.     ______________________________________

Example 1

Preparation and characterization of precipitated silicas according to the invention in the CTAB surface area range 200-250 m² /g.

50 m³ of hot water and enough commercially available sulphuric acid (96% strength) to reach a pH of 5.5 are initially placed, with stirring, in a vat which is equipped with an Ekato disc agitator and an Ekato MIG stirrer (multi-stage impulse ribbon blender). While maintaining a constant precipitation temperature of 78° C. and a pH of 5.5, 7.6 m³ /h of commercially available soda water glass solution (weight modulus 3.42, density 1.348) and 0.7 m³ /h of sulphuric acid (96% strength) are simultaneously added over the course of 70 minutes. The Ekato SR disc agitator (diameter 320 mm, 6 lamella, 740 rpm) is in operation over the whole precipitation time. After 12 minutes, addition of the reactants soda water glass solution and sulphuric acid is interrupted for 80 minutes. The two stirrers are also kept in operation during this interval. The precipitated silica is separated on a filter press, washed and the filter cakes are subjected to spray-drying or spin-flash drying and optionally granulated in the dry state between two rollers.

The precipitated silica according to the invention has a CTAB surface area of 206 m² /g, a DBP index of 298 ml/100 g and a silanol group density (V₂) of 20.5 ml. The macropore size distribution is as follows:

    ______________________________________     Macropore size interval                      Hg consumed      nm!             ml/g of silica     ______________________________________     10-20            0.29     20-30            0.24     30-40            0.19     40-50            0.15     50-60            0.11     ______________________________________

EXAMPLE 2

50 m³ of hot water and enough commercially available sulphuric acid (96 % strength) to reach a pH of 5.5 are initially placed, with stirring, in a vat which is equipped with an Ekato disc agitator and an Ekato MIG stirrer. While maintaining a constant precipitation temperature of 78° C. and a pH of 5.5, 7.6 m³ /h of commercially available soda water glass solution (weight modulus 3.42, density 1.348) and 0.7 m³ /h of sulphuric acid (96 % strength) are simultaneously added over the course of 56 minutes. The Ekato SR disc agitator (diameter 320 mm, 6 lamella, 740 rpm) is in operation over the whole precipitation time. After 12 minutes, addition of the reactants soda water glass solution and sulphuric acid is interrupted for 80 minutes. The two stirrers are also kept in operation during this interval. The precipitated silica is separated on a filter press, washed and the filter cakes are subjected to spray-drying or spin-flash drying and optionally granulated in the dry state between two rollers.

The precipitated silica according to the invention has a CTAB surface area of 240 m² /g, a DBP index of 314 ml/100 g and a silanol group density (V₂) of 21.0 ml. The macropore size distribution is as follows:

    ______________________________________     Macropore size interval                      Hg consumed      nm!             ml/g of silica     ______________________________________     10-20            0.48     20-30            0.30     30-40            0.16     40-50            0.13     50-60            0.09     ______________________________________

Example 3

Production and characterization of a precipitated silica according to the invention with a CTAB surface area in the range 250-300 m² /g

57 m³ of hot water and enough commercially available sulphuric acid (96 % strength) to reach a pH of 5.5 are initially placed, with stirring, in a vat which is equipped with an Ekato disc agitator and an Ekato MIG stirrer. While maintaining a constant precipitation temperature of 55° C. and a pH of 5.5, 8.2 m³ /h of commercially available soda water glass solution (weight modulus 3.42, density 1.348) and 0.75 m³ /h of sulphuric acid (96 % strength) are simultaneously added over the course of 56 minutes. The Ekato SR disc agitator (diameter 320 mm, 6 lamella, 740 rpm) is in operation over the whole precipitation time. After 13 minutes, addition of the reactants soda water glass solution and sulphuric acid is interrupted for 90 minutes. The two stirrers are also kept in operation during this interval. The precipitated silica is separated on a filter press, washed and the filter cakes are subjected to spray-drying or spin-flash drying and optionally granulated in the dry state between two rollers.

The precipitated silica according to the invention has a CTAB surface area of 283 m² /g, a DBP index of 349 ml/100 g and a silanol group density (V₂) of 24.9 ml. The macropore size distribution is as follows:

    ______________________________________     Macropore size interval                      Hg consumed      nm!             ml/g of silica     ______________________________________     10-20            0.42     20-30            0.22     30-40            0.14     40-50            0.10     50-60            0.07     ______________________________________

Example 4

Production and characterization of a precipitated silica according to the invention with a CTAB surface area in the range 300-400 m² /g

57 m³ of hot water and enough commercially available sulphuric acid (96 % strength) to reach a pH of 5.5 are initially placed, with stirring, in a vat which is equipped with an Ekato disc agitator and an Ekato MIG stirrer. While maintaining a constant precipitation temperature of 50° C. and a pH of 5.5, 8.2 m³ /h of commercially available soda water glass solution (weight modulus 3.42, density 1.348) and 0.75 m³ /h of sulphuric acid (96 % strength) are simultaneously added over the course of 56 minutes. The Ekato SR disc agitator (diameter 320 mm, 6 lamella, 740 rpm) is in operation over the whole precipitation time. After 13 minutes, addition of the reactants soda water glass solution and sulphuric acid is interrupted for 90 minutes. The two stirrers are also kept in operation during this interval. The precipitated silica is separated on a filter press, washed and the filter cakes are subjected to spray-drying or spin-flash drying and optionally granulated in the dry state between two rollers.

The precipitated silica according to the invention has a CTAB surface area of 360 m² /g, a DBP index of 350 ml/100 g and a silanol group density (V₂) of 29.0 ml. The macropore size distribution is as follows:

    ______________________________________     Macropore size interval                      Hg consumed      nm!             ml/g of silica     ______________________________________     10-20            0.35     20-30            0.19     30-40            0.13     40-50            0.11     50-60            0.09     ______________________________________

The physico-chemical characteristics of the precipitated silica granules obtained in accordance with examples 1 to 4 are as follows:

    ______________________________________                    Example                    1    2        3      4     ______________________________________     Moisture content %                      4.8    5.5      5.0  5.2     N.sub.2 surface area (BET) m.sup.2 /g                      275    290      413  522     Compacted density g/l                      246    270      234  220     Absorption of DBP g/100 g                      194    198      226  236     by original material     ALPINE screen tailings >63 μm                      89     93       83   85     ______________________________________

In carrying out the invention to produce the new precipitated silicas, the total precipitation time can range from about 30 to 90 minutes, not counting the time of interruption which can range from 30 to 120 minutes. The above examples show a total precipitation time of 56 to 70 minutes. In all examples, the first precipitation step is halted when it is detected that the viscosity increase. After the interruption time of 30 to 120 minutes, the precipitation process is taken up again with the addition of the reactants.

Example 5

Precipitated silicas in a NR truck vehicle tread compound

    ______________________________________     Formula     (Standard)    2       3         4     5     ______________________________________     RSS 1  100        100     100     100   100     N 220  50         --      --      --    --     Pptd.  --         38      --      --    --     silica     from ex. 1     Pptd.  --         --      35      --    --     silica     from ex. 2     Pptd.  --         --      --      32    --     silica     from ex. 3     Pptd.  --         --      --      --    32     silica     from ex. 4     ZnO RS 4          4       4       4     4     Stearic            2          2       2       2     2     acid     6PPD   2          2       2       2     2     TMQ    1          1       1       1     1     Protector            1.5        1.5     1.5     1.5   1.5     G 35     X 50-S --         19      19      19    19     TBBS   1.2        --      --      --    --     DPG    --         0.75    0.75    0.75  0.75     CBS    --         1.45    1.45    1.45  1.45     Sulphur            1.2        1.7     1.7     1.7   1.7     ______________________________________

Mixing procedure

Mixer: GK 1.6N

Step 1

Friction: 1:1.1

Speed of rotation: 70

Extent of filling: 0.7

    ______________________________________     Mixing time Mixture 1     Mixture 2-5     ______________________________________     0-1'        Polymer       Polymer     1-2'        1/2 N220, ZnO,                               1/2 silica, ZnO,                 stearic acid, stearic acid,                 protector G 35                               protector G 35     2-3'        1/2 N 220     1/2 silica,                 6PPD + TMQ    6PPD + TMQ     3-4.5'      Mix and discharge                               Mix and discharge     Discharge temperature: 155° C.     ______________________________________

Step 2

Friction: 1:1.1

Speed of rotation: 70

Extent of filling: 0.68

    ______________________________________     Mixing time    Mixture 1    Mixture 2-5     ______________________________________     0-4'           Step 1 batch Step 1 batch,                                 X 50-S     4'             Discharge     Discharge temperature: 160° C.     ______________________________________

Step 3

Friction: 1:1.1

Speed of rotation: 40

Extent of filling: 0.65

    ______________________________________     Mixing time Mixture 1     Mixture 2-5     ______________________________________     0-1.5'      Step 2 batch  Step 2 batch                 TBBS, sulphur CBS, DPG, sulphur     1.5'        Discharge     Discharge temperature: ˜90° C.     ______________________________________

Vulcanizate 150° C./t_(95%)

    ______________________________________     Mixture                  2        3        4      5                  Pptd.    Pptd.    Pptd.  Pptd.                  silica   silica   silica silica     1            from     from     from   from     standard     ex. 1    ex. 2    ex. 3  ex. 4     ______________________________________     Vulcanizing             20       35       35     35     40     time  min!     F.sup.2 H (dis-             13897    10327    6688   12130  13472     persion)     ML (1 + 4)             71       71       68     71     68     Tensile 25.0     23.6     25.9   23.4   25.9     strength      MPa!     300%    11.7     12.5     12.0   11.2   10.9     modulus      MPa!     Shore A 66       69       69     68     64     hardness     DIN     98       103      91     103    89     abrasion      mm3!     Tear    26       25       28     30     34     propagation     resistance      N/mm!     MTS     0.133    0.069    0.068  0.073  0.065     tan ∂     (60° C.)     ______________________________________

Use of the precipitated silicas according to the invention leads to a much lower tan δ (60° C.) value for approximately the same hardness and abrasion values, which means that the rolling resistance, and thus the gasoline consumption, can be clearly reduced as compared with a standard truck tread compound.

The dispersion, determined by a roughness measurement, was of the same order as in the carbon black standard sample, despite the large surface area of the precipitated silicas.

Example 6

Precipitated silicas in a NR/BR truck vehicle tread compound

    ______________________________________              Formulation              1              (Standard)                       2      3       4    5     ______________________________________     RSS 1      100        75     75    75   75     Buna CB 11S                --         25     25    25   25     N 220      50         --     --    --   --     Pptd. silica                --         38     --    --   --     from ex. 1     Pptd. silica                --         --     35    --   --     from ex. 2     Pptd. silica                --         --     --    32   --     from ex. 3     Pptd. silica                --         --     --    --   32     from ex. 4     ZnO RS     4          4      4     4    4     Stearic acid                2          2      2     2    2     6PPD       2          2      2     2    2     TMQ        1          1      1     1    1     Protector G                1.5        1.5    1.5   1.5  1.5     35     X 50-S     --         19     19    19   19     TBBS       1.2        --     --    --   --     DPG        --         0.75   0.75  0.75 0.75     CBS        --         2      2     2    2     Sulphur    1.2        1.7    1.7   1.7  1.7     ______________________________________

Mixing procedure: see example 5

Vulcanizate data: 150° C./t_(95%)

    __________________________________________________________________________                Mixture                     2     3     4     5                1    Pptd. silica                           Pptd. silica                                 Pptd. silica                                       Pptd. silica                standard                     from ex. 1                           from ex. 2                                 from ex. 3                                       from ex. 4     __________________________________________________________________________     Vulcanizing time                20   30    30    30    30      min!     F.sup.2 H (dispersion)                14721                     12398 12321 14098 10876     ML (1 + 4) 72   79    84    84    87     Tensile strength  MPa!                25.7 22.3  22.6  22.9  20.7     300% modulus  MPa!                11.7 13.6  12.5  12.2  11.5     Elongation at break  %!                540  430   450   470   457     Shore A hardness                67   71    70    70    69     DIN abrasion  mm3!                95   70    73    83    70     MTS        0.150                     0.063 0.063 0.062 0.062     tan ∂ (60° C.)     __________________________________________________________________________

The mixtures with precipitated silicas according to the invention, with approximately identical values for hardness and modulus, have an excellent dispersion, good resistance to abrasion and a very low tan δ (60° C.) value (equivalent to rolling resistance).

Example 7

Precipitated silicas in a NR/S-SBR tread compound

    ______________________________________              Formulation              1              (Standard)                       2      3       4    5     ______________________________________     RSS 1      100        75     75    75   75     Buna VSL   --         25     25    25   25     1954/S 25     N 220      50         --     --    --   --     Pptd. silica                --         38     --    --   --     from ex. 1     Pptd. silica                --         --     35    --   --     from ex. 2     Pptd. silica                --         --     --    32   --     from ex. 3     Pptd. silica                --         --     --    --   32     from ex. 4     ZnO RS     4          4      4     4    4     Stearic acid                2          2      2     2    2     6PPD       2          2      2     2    2     TMQ        1          1      1     1    1     Protector G                1.5        1.5    1.5   1.5  1.5     35     X 50-S     --         19     19    19   19     TBBS       1.2        --     --    --   --     DPG        --         0.75   0.75  0.75 0.75     CBS        --         2      2     2    2     Sulphur    1.2        1.3    1.3   1.3  1.3     ______________________________________

Mixing procedure: see example 5

Vulcanizate data: 150° C./t_(95%)

    __________________________________________________________________________                Mixture                     2     3     4     5                1    Pptd. silica                           Pptd. silica                                 Pptd. silica                                       Pptd. silica                standard                     from ex. 1                           from ex. 2                                 from ex. 3                                       from ex. 4     __________________________________________________________________________     Vulcanizing time                20   25    25    25    25      min!     F.sup.2 H (dispersion)                12738                     17520 20196 27299 17226     ML (1 + 4) 71   71    68    71    71     Tensile strength  MPa!                24.7 18.6  21.3  20.3  20.3     300% modulus  MPa!                11.4 15.1  13.4  11.8  10.6     Elongation at break  %!                530  360   430   450   471     Shore A hardness                66   71    69    67    66     DIN abrasion  mm3!                110  106   107   110   115     MTS     tan ∂ (0° C.)                0.260                     0.270 0.264 0.256 0.265                                       (0.165)     tan ∂ (60° C.)                0.133                     0.073 0.082 0.082 0.080     __________________________________________________________________________

Use of the precipitated silicas according to the invention, with excellent dispersion and comparable abrasion to that of a standard mixture, can both improve tan δ (0° C.) (equivalent to skid resistance in the wet) and also reduce the rolling resistance tan δ (60° C.)!.

Further variations and modifications will be apparent to those skilled in the art from the foregoing and are intended to be encompassed by the claims appended hereto. German priority applications 195 20 126.4 and 196 17 039.7 are relied on and incorporated herein by reference. 

We claim:
 1. A process for producing a precipitated silica modified with an organosilane, said silica having a CTAB surface area, in accordance with ASTM D 3765-92, of 200 to 400 m² /g, a DBP index, in accordance with ASTM D 2414, between 230 and 380 ml/100 g as powder and 180-250 g/100 g as granulate, a silanol group density, V₂ -NaOH consumption, of 20 to 30 ml and the following macroproe size distribution which is typical of the surface area range involved, determined by means of Hg porosimetry (DIN 66 133) for specific pore size intervals (incremental mode of application):

    ______________________________________               CTAB surface                        CTAB surface                                   CTAB surface               area range:                        area range:                                   area range:               200-250 m.sup.2 /g                        250-300 m.sup.2 /g                                   300-400 m.sup.2 /g     Pore size interval in     nm          Hg consumption in ml/g of silica     ______________________________________     10-20       0.27-0.49  0.35-0.50  0.32-0.42     20-30       0.22-0.32  0.15-0.30  0.17-0.22     30-40       0.15-0.21  0.12-0.17  0.12-0.15     40-50       0.11-0.16  0.09-0.12  0.08-0.11     50-60       0.08-0.12  0.06-0.10  0.06-0.09     ______________________________________

and produced by reacting sulphuric acid with an alkali metal silicate solution, which process comprises: acidifying water at a temperature from 30° to 90° C. to pH of 5 to 5.9 by adding small amounts of sulphuric acid; introducing simultaneously said alkali metal silicate solution and sulphuric acid into said acidified water to precipitate silica, the pH of the solution being kept constant at 5 to 5.9 and the reaction solution being agitated during the entire process; suspending the introduction of said alkali metal silicate and sulphuric acid when it is detected that the viscosity of the reaction solution increases to halt the precipitation of silica; after an interval of 30 to 120 minutes, resuming the introduction of alkali metal silicate and sulphuric acid into the reaction solution until the concentration of silica reaches from 40 to 60 g/liter, thereby producing a silica suspension; and further reacting said precipitated silica with an organosilane of the formula I, II or III:

     R.sub.n.sup.1 --(RO).sub.3-n Si-(alk).sub.m -(ar).sub.p !.sub.q  B!(I)

or

    R.sub.n.sup.1 (RO).sub.3-n Si-(alkyl)                      (II)

or

    R.sub.n.sup.1 (RO).sub.3-n Si(alkenyl)                     (III)

wherein B represents --SCN, --SH, --Cl, --NH₂ (if q=1) or --S_(x) -- (if q=2) R and R¹ represent an alkyl group with 1 to 4 carbon atoms or a phenyl group, wherein all R and R¹ groups may be identical or different, n is 0, 1 or 2, alk represents a divalent, straight-chain or branched hydrocarbon group with 1 to 6 carbon atoms, m is 0 or 1, ar represents an arylene group with 6 to 12 carbon atoms, p is 0 or 1, with the proviso that p and n are not simultaneously 0, x is a number from 2 to 8, alkyl represents a monovalent, straight-chain or branched saturated hydrocarbon group with 1 to 20 carbon atoms, alkenyl represents a monovalent, straight-chain or branched unsaturated hydrocarbon group with 2 to 20 carbon atoms.
 2. The process according to claim 1 where ar is 6 carbon atoms.
 3. The process according to claim 1 wherein alkyl contains 2 to 8 carbon atoms.
 4. The process according to claim 1 where alkenyl contains 2 to 8 carbon atoms.
 5. A vulcanizable rubber mixture comprising, based on 100 parts of Rubber, from 5 to 100 parts of a precipitated silica having a CTAB surface area, in accordance with ASTM D 3765-92, of 200 to 400m² /g, a DBP index, in accordance with ASTM D 2414, between 230 and 380 ml/100 g as powder and 180-250 g/100 g as granulate, a silanol group density, V₂ -NaOH consumption, of 20 to 30 ml and the following macroproe size distribution which is typical of the surface area range involved, determined by means of Hg porosimetry (DIN 66 133) for specific pore size intervals (incremental mode of application):

    ______________________________________               CTAB surface                        CTAB surface                                   CTAB surface               area range:                        area range:                                   area range:               200-250 m.sup.2 /g                        250-300 m.sup.2 /g                                   300-400 m.sup.2 /g     Pore size interval  nm!                 Hg consumption in ml/g of silica     ______________________________________     10-20       0.27-0.49  0.35-0.50  0.32-0.42     20-30       0.22-0.32  0.15-0.30  0.17-0.22     30-40       0.15-0.21  0.12-0.17  0.12-0.15     40-50       0.11-0.16  0.09-0.12  0.08-0.11     50-60       0.08-0.12  0.06-0.10  0.06-0.09     ______________________________________

and produced by reacting sulphuric acid with an alkali metal silicate solution, which process comprises: acidifying water at a temperature from 30° to 90° C. to a pH of 5 to 5.9 by adding small amounts of sulphuric acid; introducing simultaneously said alkali metal silicate solution and sulphuric acid in to said acidified water to precipitate silica, the pH of the solution being kept constant at 5 to 5.9 and the reaction solution being agitated during the entire process; suspending the introduction of said alkali metal silicate and sulphuric acid when it is detected that the viscosity of the reaction solution increases to halt the precipitation of silica; after an interval of 30 to 120 minutes, resuming the introduction of alkali metal silicate and sulphuric acid into the reaction solution until the concentration of silica reaches from 40 to 60 g/liter, thereby producing a silica suspension.
 6. The vulcanizable rubber mixture according to claim 5 comprising 15 to 60 parts of said precipitated silica.
 7. A precipitated silica having its surface modified with an organolsilane, wherein said silica has a CTAB surface area, in accordance with ASTM D 3765-92, of 200 to 400 m² /g, a DBP index, in accordance with ASTM D 2414, between 230 and 380 ml/100 g as powder and 180-250 g/100 g as granulate, a silanol group density, V₂ -NaOH consumption, of 20 to 30 ml and the following macroproe size distribution which is typical of the surface area range involved, determined by means of Hg porosimetry (DIN 66 133) for specific pore size intervals (incremental mode of application):

    ______________________________________               CTAB surface                        CTAB surface                                   CTAB surface               area range:                        area range:                                   area range:               200-250 m.sup.2 /g                        250-300 m.sup.2 /g                                   300-400 m.sup.2 /g     Pore size interval in     nm          Hg consumption in ml/g of silica     ______________________________________     10-20       0.27-0.49  0.35-0.50  0.32-0.42     20-30       0.22-0.32  0.15-0.30  0.17-0.22     30-40       0.15-0.21  0.12-0.17  0.12-0.15     40-50       0.11-0.16  0.09-0.12  0.08-0.11     50-60       0.08-0.12  0.06-0.10  0.06-0.09     ______________________________________

and produced by reacting sulphuric acid with an alkali metal silicate solution, which process comprises: acidifying water at a temperature from 30° to 90° C. to a pH of 5 to 5.9 by adding small amounts of sulphuric acid; introducing simultaneously said alkali metal silicate solution and sulphuric acid in to said acidified water to precipitate silica, the pH of the solution being kept constant at 5 to 5.9 and the reaction solution being agitated during the entire process; suspending the introduction of said alkali metal silicate and sulphuric acid when it is detected that the viscosity of the reaction solution increases to halt the precipitation of silica; after an interval of 30 to 120 minutes, resuming the introduction of alkali metal silicate and sulphuric acid into the reaction solution until the concentration of silica reaches from 40 to 60 g/liter, thereby producing a silica suspension, wherein said organosilane is a member selected from the group consisting of the formulas I, II and III

     R.sub.n.sup.1 -(RO).sub.3-n Si-(alk).sub.m -(ar).sub.p !.sub.q  B!(I)

or

    R.sub.n.sup.1 (RO).sub.3-n Si-(alkyl)                      (II)

    R.sub.n.sup.1 (RO).sub.3-n Si(alkenyl)                     (III)

wherein B represents --SCN, --SH, --Cl, --NH₂ (if q=1) or --X_(x) -- (if q=2) R and R¹ represent an alkyl group with 1 to 4 carbon atoms or a phenyl group, wherein all R and R¹ groups may be identical or different, n is 0, 1 or 2, alk represents a divalent, straight-chain or branched hydrocarbon group with 1 to 6 carbon atoms, m is 0 or 1, ar represents an arylene group with 6 to 12 carbon atoms, p is 0 or 1, with the proviso that p and n are not simultaneously 0, x is a number from 2 to 8, alkyl represents a monovalent, straight-chain or branched saturated hydrocarbon group with 1 to 20 carbon atoms, alkenyl represents a monovalent, straight-chain or branched unsaturated hydrocarbon group with 2 to 20 carbon atoms.
 8. The precipitated silica according to claim 7 wherein ar contains 6 carbon atoms.
 9. The precipitated silica according to claim 7 wherein alkyl contains 2 to 8 carbon atoms.
 10. The precipitated silica according to claim 7 wherein alkenyl contains 2 to 8 carbon atoms.
 11. The vulcanizable rubber mixture according to claim 5 which is modified with organosilanes.
 12. A vulcanized rubber mixture made from the vulcanizable rubber mixture according to claim 5 optionally modified with organosilianes. 